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## Tips & Tricks: Inflation Layer Meshing in ANSYS

Throughout our first set of Tips & Tricks posts relating to meshing controls and meshing methods, we have made mention of Inflation Layers a number of times.  Let's take this opportunity to explain exactly why inflation layers are a critical component of a good CFD mesh and how we can create them easily within ANSYS Meshing. In our first posts on Mesh Sizing we explained that as well as capturing all key features of the geometry (using local sizing and the curvature size function), we also need to have a sufficiently fine mesh to adequately capture regions where the flow will experience rapid change in key variables such as pressure, velocity or temperature.   This initially led us to a better understanding of how we should apply Global and Local Mesh Controls. Now, if you think about moving a probe from the freestream flow towards one of the walls in your fluid domain, as you approach the wall you will notice that the velocity decreases non-linearly up to a point where the fluid will have zero velocity at the wall.  This is what is termed the "no slip" wall condition in CFD. If we plot a typical velocity profile in the near-wall region, we can see that we have a large change in velocity in the wall normal direction and it is important to our CFD simulation that we capture this gradient correctly.  To do this, we need to use inflation layer meshing to accurately capture the boundary layer region for any wall-bounded turbulent flows.  The image below plots the non-dimensional velocity versus the non-dimensional wall normal distance, with each line from top to bottom demonstrating the difference between a favourable pressure gradient through to adverse pressure gradient with flow separation.  It is clear that the flow behaviour in the near wall region is fairly complex and needs to be captured appropriately to have any confidence in our CFD results, especially if we intend to report key engineering data such as separation points or pressure drops.                     Providing a suitable inflation mesh for the geometry is strongly tied to the choice of the turbulence model, and the flow field we are interested in capturing. We can elect to resolve the complete profile of the boundary layer of alternatively we can make use of empirical wall functions to reduce the cell count (see our post on turbulence modelling and wall functions). If we refer to the images below, on the left hand side we observe that the boundary layer profile is modelled with a reduced cell count, which is characteristic of a wall function approach. On the right, the boundary layer profile is resolved all the way to the wall....